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Structure Sensitivity of the Reaction of Methanol on Ceria R. M. Ferrizz,† G. S. Wong,† T. Egami,‡ and J. M. Vohs*,† Department of Chemical Engineering and Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Received December 11, 2000 Temperature programmed desorption was used to study the reaction of CH3OH on several different ceria-based model catalysts. These catalysts consisted of a CeO2(111) single crystal and thin ceria films supported on R-Al2O3(0001) and yttria-stabilized zirconia (100). The results of this study demonstrate that the reaction of CH3OH on CeO2 surfaces is highly structure sensitive and depends on crystallographic orientation, the concentration of surface oxygen vacancies, and the oxidation state of surface cerium cations. The primary decomposition pathway for methoxide intermediates adsorbed on surface oxygen vacancy sites is dehydrogenation to produce H2CO and surface hydroxyl groups. The surface hydroxyl groups then either react with additional methoxides to reform CH3OH or react to produce H2O. In contrast, methoxides adsorbed on partially reduced ceria surfaces, possibly on Ce3+ sites, undergo complete dehydrogenation to CO and H2.
Introduction Ceria composes a major component in the three-way catalyst for automotive exhaust pollution control. The primary function of ceria in these catalysts is to dampen out fluctuations in the oxygen partial pressure and maintain the air-to-fuel ratio near the value required for optimal catalyst performance for both CO and hydrocarbon oxidation and NOx reduction.1-8 This function, which is often referred to as oxygen storage, relies on the fact that ceria has multiple stable oxidation states. Under fuelrich conditions, Ce4+ cations are reduced to Ce3+ and release oxygen from the lattice, whereas under fuel-lean conditions this process is reversed. Numerous studies have shown that the oxygen storage capacity of ceria can be enhanced by mixing with zirconia.8-12 To better understand the mechanism of oxygen storage and the promotional effect of zirconia, we have been studying the reactivity of model ceria catalysts consisting of CeO2 single crystals and CeO2 thin films supported on both R-Al2O3(0001) and yttria-stabilized zirconia (100) (YSZ(100)) single crystals.8,13-20 These studies have shown that interactions between ceria and zirconia significantly †
Department of Chemical Engineering. ‡ Department of Materials Science and Engineering. (1) Trovarelli, A. Catal. Rev. 1996, 38, 439. (2) Taylor, K. C. Catal. Rev. 1993, 35, 457. (3) McCabe, R. W.; Kisenyi, J. M. Chem. Ind. 1995, 15, 605. (4) Jen, H. W.; Graham, G. W.; Chun, W.; McCabe, R. W.; Cuif, J. P.; Deutsch, S. E.; Touret, O. Catal. Today 1999, 50, 309. (5) Nunan, J. G.; Robota, H. C.; Cohn, M. J.; Bradley, S. A. J. Catal. 1992, 133, 309. (6) Shelef, M.; Graham, G. W. Catal. Rev. 1994, 36, 433. (7) Gandhi, H. S.; Shelef, M. Stud. Surf. Sci. Catal. 1987, 30, 199. (8) Putna, E. S.; Bunluesin, T.; Fax, X. L.; Gorte, R. J.; Vohs, J. M.; Lakis, R. E.; Egami, T. Catal. Today 1999, 50, 343. (9) Ohatu, T. Rare Earths 1990, 17, 37. (10) Murota, T.; Hasegawa, T.; Aozasa, S.; Matsui, H.; Motoyama, M. J. Alloys Compd. 1993, 193, 298. (11) Haneda, M.; Miki, K.; Katusa, N.; Ueno, A.; Matsuura, S.; Sato, M. Nihon Kagaku Kaishi 1990, 820. (12) Fornasiero, P.; Monte, R. D.; Rao, G. R.; Kaspar, J.; Meriani, S.; Trovarelli, A.; Graziani, M. J. Catal. 1995, 151, 168. (13) Putna, E. S.; Vohs, J. M.; Gorte, R. J. J. Phys. Chem. 1996, 100, 17862. (14) Putna, E. S.; Gorte, R. J.; Vohs, J. M. J. Catal. 1998, 178, 598. (15) Dmowski, W.; Egami, T.; Gorte, R. J.; Vohs, J. M. Physica B 1996, 221, 420. (16) Ferrizz, R. M.; Egami, T.; Vohs, J. M. Catal. Lett. 1999, 61, 33. (17) Ferrizz, R. M.; Egami, T.; Vohs, J. M. Surf. Sci. 2000, 465, 127.
enhance the reducibility of the ceria. Ceria thin films supported on YSZ(100) contain a high number of oxygen vacancies and are less thermally stable than bulk single crystals or CeO2 films supported on R-Al2O3(0001).8,13-20 In addition to the differences in redox behavior, these ceria model catalysts have also been shown to have different reactivities toward NO.18 Furthermore, it has been shown that the reactivity differences are due to variations in the concentration and type of oxygen vacancies on each surface. These model systems, therefore, provide excellent test beds for studying how surface oxygen vacancies influence the reactivity of ceria. In the work presented here, we have extended our previous studies with NO to include the reaction of methanol on the model CeO2 catalysts. The temperature programmed desorption (TPD) results presented below show that surface oxygen vacancies are the active sites for the dissociative adsorption of CH3OH on ceria and their concentration influences the reaction pathways for adsorbed methoxide intermediates. Experimental Section The experimental procedures used in this study were similar to those described in our previous investigations of the reactivity of ceria thin films.16-18 Growth of the ceria films and the TPD experiments were conducted in a single ultrahigh vacuum (UHV) surface analysis system. This system has a background pressure of 2 × 10-10 Torr and is equipped with a mass spectrometer (UTI), cylindrical mirror electron energy analyzer (Omicron), ion sputter gun (Physical Electronics), electron gun, quartz crystal film thickness monitor (Maxtek), and metal deposition sources. The R-Al2O3(0001), YSZ(100), and CeO2(111) substrates were mounted on a UHV sample manipulator using a sample holder made out of tantalum foil. Resistive heating of the tantalum holder allowed the samples to be heated in excess of 1000 K. The sample temperature was monitored using a type K thermocouple that was glued to the back surface of the sample using a ceramic adhesive. Each oxide substrate was cleaned via sputtering with 2 keV Ar+ ions followed by annealing at 800 K for 60 min. This procedure was repeated until the surface was free from impurities as determined by Auger electron spectroscopy (AES). It has previously been shown that CeO2(111) surfaces prepared in this (18) Ferrizz, R. M.; Egami, T.; Wong, G. S.; Vohs, J. M. Surf. Sci., in press. (19) Stubenrauch, J.; Vohs, J. M. J. Catal. 1996, 159, 50. (20) Stubenrauch, J.; Vohs, J. M. Catal. Lett. 1997, 47, 21.
10.1021/la001729o CCC: $20.00 © 2001 American Chemical Society Published on Web 03/15/2001
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Figure 1. TPD spectra obtained from (a) CH3OH-dosed vacuum-annealed CeO2(111) and (b) CH3OH-dosed Ar+-sputtered CeO2(111). manner exhibit a (1 × 1) hexagonal low-energy electron diffraction (LEED) pattern.19 Because of sample charging, LEED analysis of the R-Al2O3(0001) and YSZ(100) surfaces was not possible. Ceria films were grown on the R-Al2O3(0001) and YSZ(100) substrates by vapor depositing cerium metal in the presence of 1 × 10-7 of Torr of O2 (Matheson, >99.6%). The evaporative cerium source consisted of a small tantalum boat filled with cerium metal, which could be heated by electron bombardment. Ceria films, 10 monolayers (∼40 Å) in thickness, as determined using a quartz crystal film thickness monitor, were used in this study. After film deposition, the sample was annealed at 450 K in 1 × 10-7 Torr of O2 for 15 min to ensure complete oxidation of the ceria layer. Methanol (Alfa Aesar, semiconductor grade) was contained in a small glass bulb on a gas manifold that was attached to the UHV chamber through a variable leak valve. The CH3OH was purified using repeated freeze-pump-thaw cycles prior to use. In each of the TPD experiments reported here, the sample was exposed to a saturation dose of CH3OH at 300 K and then heated at a rate of 4.5 K/s. The mass spectrometer was computer multiplexed, and multiple m/e values were monitored during each TPD run. The relative yields of the various products detected in the TPD experiments were determined using the method described in ref 21.
Results 1. CeO2(111). TPD spectra for methanol (CH3OH) and formaldehyde (H2CO) obtained following exposure of an annealed CeO2(111) surface to a saturation dose of CH3OH at 300 K are displayed in Figure 1a. Upon heating, CH3OH desorbed in a broad feature spanning from 360 to 750 K. A narrow peak centered at 310 K was also observed which can be attributed to desorption from the crystal support hardware. The H2CO desorption spectrum contained a small peak centered near 680 K. Formaldehyde was the only decomposition product detected during this TPD run. On the basis of the peak areas, the total amount of CH3OH that adsorbed on the annealed CeO2(111) surface was estimated to be less than 0.08 monolayers (ML). This result demonstrates that the annealed (21) Ko, E. I.; Benziger, J. B.; Madix, R. J. J. Catal. 1980, 62, 264.
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Figure 2. CH3OH desorption spectra from a series of TPD experiments with CH3OH-dosed CeO2/YSZ(100). A freshly prepared CeO2/YSZ(100) sample was used in the first TPD run in the series.
CeO2(111) surface is not very active for the dissociative adsorption of methanol at room temperature. The adsorption and reaction of CH3OH on this surface most likely occurs at surface defect sites. Methanol TPD spectra were also obtained for CeO2(111) surfaces that were sputtered with 0.5 keV Ar+ ions for 15 min. It has previously been shown that sputtering with Ar+ preferentially removes oxygen from the CeO2(111) surface.19 Thus, the sputtered surface contains a high number of oxygen vacancies. Figure 1b displays the CH3OH and H2CO desorption spectra from a TPD run with a sputtered surface. Formaldehyde was again the only decomposition product detected. The CH3OH desorption spectrum contains a broad, double peak feature spanning 375-715 K, with peak maxima at 565 and 670 K, and the H2CO desorption spectrum contains a single peak at 675 K. The desorption spectrum for water continually increased throughout the TPD run but did not contain any distinct peaks. Note that the total amount of CH3OH that adsorbed on the sputtered surface was an order of magnitude greater than that on the annealed surface. Several additional TPD experiments were performed with the sputtered CeO2(111) sample, and with the exception of a slight decrease in the intensity of the lower temperature methanol desorption peak, the results were identical to those shown in Figure 1b. 2. CeO2/YSZ(100). In previous studies, we have shown that vapor-deposited CeO2 films grow epitaxially on YSZ(100) with a (100) surface orientation.15 It has also been shown that for the growth conditions used in this study, the YSZ(100)-supported CeO2 films are not fully oxidized, and a small fraction of the cerium cations are in the +3 oxidation state.18 Furthermore, these films have been shown to be thermally unstable and undergo significant reduction upon heating to 900 K.16-18 Desorption spectra for CH3OH, H2CO, CO, H2O, and H2 obtained in a series of CH3OH TPD runs with a freshly prepared CeO2/YSZ(100) sample are presented in Figures 2-6, respectively. To limit the thermally induced reduction of the ceria film, the sample was heated to only 750 K in the first two TPD runs. In subsequent runs, the sample
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Figure 3. H2CO desorption spectra from a series of TPD experiments with CH3OH-dosed CeO2/YSZ(100). A freshly prepared CeO2/YSZ(100) sample was used in the first TPD run in the series.
Figure 5. H2O desorption spectra from a series of TPD experiments with CH3OH-dosed CeO2/YSZ(100). A freshly prepared CeO2/YSZ(100) sample was used in the first TPD run in the series.
Figure 4. CO desorption spectra from a series of TPD experiments with CH3OH-dosed CeO2/YSZ(100). A freshly prepared CeO2/YSZ(100) sample was used in the first TPD run in the series.
Figure 6. H2 desorption spectra from a series of TPD experiments with CH3OH-dosed CeO2/YSZ(100). A freshly prepared CeO2/YSZ(100) sample was used in the first TPD run in the series.
was heated to 900 K. In the first CH3OH TPD run with the freshly prepared CeO2/YSZ(100) sample, CH3OH desorbed between 400 and 660 K. The desorption curve appears to be composed of several overlapping peaks, the largest of which is centered at 620 K. The spectrum for H2CO contains a single peak at 635 K, and the spectrum for CO contains a peak at 645 K. This trend in desorption temperatures (i.e., TCO > TH2CO > TCH3OH) was observed in all the TPD runs for both the CeO2/YSZ(100) and CeO2/ R-Al2O3(0001) samples. The H2O desorption curve for run 1 contains a broad peak at low temperature and a more
distinct feature near 645 K. The spectrum for H2 also contains a peak centered near 645 K. A slight increase in the desorption temperatures for the decomposition products occurred in each successive run in the TPD series. Note that despite this temperature shift from run to run, H2, H2O, and CO are always produced at the same temperature, H2CO is produced at a slightly lower temperature, and CH3OH desorbs at an even lower temperature than H2CO. The relative yields of the C1 products for each run in the series are listed in Table 1. The relative amount of CH3OH that adsorbed in each run
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Table 1. C1 Product Yields for CH3OH TPD from CeO2/YSZ(100) run no.
relative amount of CH3OH adsorbed
1 2 3 4
0.77 0.87 1.00 0.63
C1 product yields CH3OH H2CO CO 0.62 0.55 0.55 0.39
0.26 0.17 0.08 0.03
0.12 0.29 0.37 0.58
is also listed in this table. For the first three runs in the series, the total amount of CH3OH that adsorbed increased with each successive run, and the ratio of H2CO to CO decreased substantially. The overall amount of H2 and H2O produced also increased over these runs. Because H2O production requires the removal of lattice oxygen, these changes can be attributed to an increase in the number of defect sites on the surface. Run 4 was the first run performed after the sample had been heated to 900 K. As noted above, heating to this temperature induces substantial reduction of the YSZ(100)-supported CeO2 film.18 Thus, this sample is more highly reduced than that used in runs 1-3. The TPD results obtained in run 4 were substantially different than those obtained in the earlier runs. Somewhat surprisingly, the total amount of methanol adsorbed in run 4 was 40% less than that in run 3. The dominant C1 desorption product in this run was CO, with the ratio of H2CO to CO being only 0.05. A similar result was observed for the ratio of H2O to H2. In fact, the high-temperature H2O peak is almost absent in the data from run 4. 3. CeO2/r-Al2O3(0001). Unlike CeO2 on YSZ(100), vapor-deposited CeO2 films do not grow epitaxially on the R-Al2O3(0001) substrate.21 CeO2 films supported on R-Al2O3(0001) are polycrystalline, and therefore a variety of crystal planes are likely to be exposed on the surface of these films. Previous X-ray photoelectron spectroscopy (XPS) studies indicate that the freshly grown ceria films on R-Al2O3(0001) are nearly stoichiometric and more thermally stable than those on YSZ(100).18 Desorption curves for CH3OH, H2CO, and CO obtained in a series of methanol TPD experiments with a freshly prepared CeO2/R-Al2O3(0001) sample are presented in Figures 7-9, respectively. In the first run in the series, the sample was relatively unreactive and very little CH3OH adsorbed. The TPD spectra contain small peaks for both CH3OH and H2CO at 580 K. The CO, H2O, and H2 desorption spectra for this run were all nearly flat. The relative yields of the C1 products are listed in Table 2. CH3OH and H2CO desorption results for successive TPD runs in the series were similar to run 1, but with a gradual trend of increasing peak size and peak temperature. Contrary to the results for CeO2/YSZ(100), as shown by the results for runs 3 and 4, heating the CeO2/R-Al2O3(0001) sample to 900 K rather than 750 K did not produce a dramatic change in the TPD results. This is consistent with previous studies which demonstrate that ceria films supported on R-Al2O3(0001) are more thermally stable than those on YSZ(100). In run 4, a small CO desorption peak begins to emerge at 600 K. This peak continues to increase in size and temperature in subsequent runs. Run 4 was also the first run for which distinct H2O and H2 peaks were detected at approximately 610 K. The ratio of the intensities of the H2 and H2O peaks was roughly 2:1 and remained constant throughout the rest of the runs in the series. As shown in Table 2, the total amount of methanol that adsorbed increased with each successive TPD run and the relative yields of CH3OH and H2CO were fairly steady. CO production was not initially observed but was roughly
Figure 7. CH3OH desorption spectra from a series of TPD experiments with CH3OH-dosed CeO2/R-Al2O3(0001). A freshly prepared CeO2/R-Al2O3(0001) sample was used in the first TPD run in the series.
Figure 8. H2CO desorption spectra from a series of TPD experiments with CH3OH-dosed CeO2/R-Al2O3(0001). A freshly prepared CeO2/R-Al2O3(0001) sample was used in the first TPD run in the series.
10% of the C1 product by run 7. For the methanol TPD runs with CeO2/R-Al2O3(0001), a difference in the desorption temperature for the C1 products was again observed, with CO consistently desorbing at a temperature approximately 10 K greater than that for H2CO and approximately 20 K greater than that for CH3OH. Discussion Previous studies of the interaction of Brønsted acids with single-crystal metal oxide surfaces have found that
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Figure 10. Schematic diagram of the CeO2(111) surface. The larger spheres are the oxygen anions, and the smaller spheres are the cerium cations.
Figure 9. CO desorption spectra from a series of TPD experiments with CH3OH-dosed CeO2/R-Al2O3(0001). A freshly prepared CeO2/R-Al2O3(0001) sample was used in the first TPD run in the series. Table 2. C1 Product Yields for CH3OH TPD from CeO2/r-Al2O3(0001) run no.
relative amount of CH3OH adsorbed
1 2 3 4 5 6 7
0.40 0.58 0.61 0.77 0.89 0.91 1.00
C1 product yields CH3OH H2CO CO 0.73 0.75 0.80 0.75 0.71 0.63 0.61
0.27 0.25 0.20 0.19 0.22 0.29 0.26
0.00 0.00 0.00 0.06 0.07 0.08 0.12
dissociative adsorption takes place at exposed surface cation-anion site pairs.22-25 Upon dissociation, the acid proton bonds to an oxygen anion forming a surface hydroxyl group and the conjugate base anion interacts with a surface cation. On oxide surfaces that do not have exposed cation-anion site pairs, such as ZnO(0001h ), only molecular adsorption of Brønsted acids is observed.24-25 LEED and scanning tunneling microscopy results in conjunction with theoretical models have shown that the structure of the CeO2(111) surface is consistent with that predicted by an ideal termination of the bulk.26,27 A schematic diagram of this surface is displayed in Figure 10. Note that the surface is composed of an outermost layer of oxygen anions and an exposed subsurface layer of cerium cations. The cerium cations are surrounded by seven oxygen anions and have one coordination vacancy relative to those in the bulk. Thus, this surface has exposed cation-anion site pairs and would be expected to be active for the dissociative adsorption of methanol and other Brønsted acids. Indeed, it has previously been shown by both TPD and high-resolution electron energy loss spectroscopy that formic acid adsorbs dissociatively on this (22) Cordatos, H.; Bunluesin, T.; Stubenrauch, J.; Vohs, J. M.; Gorte, R. J. J. Phys. Chem. 1996, 100, 785. (23) Gercher, V. A.; Cox, D. F.; Themlin, J. M. Surf. Sci. 1994, 306, 279. (24) Vohs, J. M.; Barteau, M. A. Surf. Sci. 1986, 176, 91. (25) Vohs, J. M.; Barteau, M. A. J. Phys. Chem. 1991, 95, 297. (26) Norenberg, H.; Briggs, G. A. D. Surf. Sci. 1998, 404, 734.
surface.28 It is therefore somewhat surprising that in the present study the vacuum-annealed CeO2(111) surface was found to be nearly inactive for the dissociative adsorption of CH3OH. This may be due in part to the fact that the cations on this surface are located in octahedral sites below three surface oxygen anions. Steric interactions between the surface oxygen anions and the methyl group in a methoxide intermediate bonded to a subsurface Ce4+ cation may destabilize methoxide intermediates on this surface. Previous XPS studies have shown that Ar+ sputtering preferentially removes oxygen anions from the CeO2(111) surface. As shown in Figure 1b, the sputtered surface exhibited significant activity for both the dissociative adsorption of CH3OH and the oxidation of CH3OH to H2CO. A comparison of the TPD results for the annealed and sputtered CeO2(111) surfaces, therefore, demonstrates that oxygen vacancies are the active sites for both dissociative adsorption of CH3OH and the oxidation of methoxides to H2CO. The fact that both H2CO and CH3OH desorbed at 675 K and H2 and H2O were not produced at this temperature is consistent with a disproportionation reaction pathway in which H2CO is formed via cleavage of a C-H bond in a methoxide intermediate with the resulting hydrogen reacting with a second methoxide to form CH3OH. It is useful to compare the results obtained in this study for a CeO2(111) single-crystal surface to those reported previously by Siokou and Nix for the reaction of CH3OH on (111) surfaces of epitaxial CeO2 thin films on Cu(111).29 During CH3OH TPD with a well-oxidized 10 ML CeO2(111) thin film, the primary decomposition product was CO which was produced at 580 K. A smaller amount of H2CO was also produced at this temperature. Similar results were reported for reduced CeO2(111) surfaces. On the basis of these results, Siokou and Nix concluded that CH3OH adsorbs dissociatively on both fully oxidized and partially reduced CeO2(111) surfaces and that the concentration of surface oxygen vacancies does not significantly influence the surface chemistry. The TPD results obtained in the present study for single-crystal CeO2(111) along with those for the CeO2 thin films are at odds with these conclusions. The results of this study clearly demonstrate that fully oxidized CeO2(111) is nearly inactive for the dissociative adsorption and oxidation of methanol. Oxygen vacancy sites on this surface, however, are active for these reactions. In the case of the Siokou (27) Conesa, J. C. Surf. Sci. 1995, 339, 337. (28) Stubenrauch, J.; Brosha, E.; Vohs, J. M. Catal. Today 1996, 28, 431.
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Figure 11. Schematic diagram of the CeO2(100) surface. The larger spheres are the oxygen anions, and the smaller spheres are the cerium cations.
and Nix study, it is likely that the observed reactivity also took place at surface defect sites. Indeed, the XPS results reported in that study suggest that even the most highly oxidized samples contained a significant fraction of Ce cations in the +3 oxidation state.29 Previous transmission electron microscopy and surface X-ray scattering studies have shown that vapor-deposited CeO2 films grow epitaxially on YSZ(100) with a (100) surface orientation.15 In the (100) direction, CeO2 is composed of alternating layers of cations and anions. Ideal bulk termination of CeO2 in the (100) direction produces a surface composed exclusively of anions or cations depending on the cleavage plane. These surfaces are not charge neutral and are therefore unstable. Theoretical calculations have shown that a charge neutral, stable structure for this surface is obtained by cleaving to produce an anion-terminated surface and then removing one-half of the oxygen anions in the outermost plane.27 As shown in Figure 11, this produces a relatively open surface containing two- and three-coordinate oxygen anions and five- and six-coordinate cerium cations. Although the CeO2 film is epitaxial with the YSZ(100) substrate, surface X-ray scattering results indicate that the lateral coherence length of the individual grains in the film is only 70 Å.15 XPS and pulsed neutron diffraction studies have also shown that interactions at the CeO2YSZ(100) interface stabilize oxygen vacancies in the CeO2 layer.18,30,31 Even after oxidation treatments in air, CeO2/ YSZ(100) samples are not completely oxidized. NO TPD results for CeO2/YSZ(100) samples have also shown that Ce3+ cations are exposed on the surface of the CeO2 thin film.18 Thus, the surface of the YSZ(100)-supported CeO2 thin film has a (100) orientation and contains a high number of defects due to oxygen vacancies and grain boundaries. It has also been shown that these films are thermally unstable and undergo additional reduction upon heating to temperatures in excess of 700 K.18 The TPD results for CeO2/YSZ(100) demonstrate that in addition to recombination and desorption as CH3OH, surface methoxides undergo dehydrogenation on this surface to produce either H2CO or CO. As shown in Table 1, the ratio of H2CO to CO produced decreased significantly with each TPD run. In the first run with the freshly grown film, the ratio was 2.2:1, whereas in the third run the ratio was only 0.2:1.0. In the fourth run, the first in which the sample had been previously heated to 900 K, nearly (29) Siokou, A.; Nix, R. M. J. Phys. Chem. B 1999, 103, 6984. (30) Mamontov, E.; Egami, T.; Brezny, R.; Korrane, M. Unpublished results. (31) Mamontov, E.; Egami, T. J. Phys. Chem. Solids 2000, 61, 1345.
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all of the surface methoxides underwent complete dehydrogenation to CO. Because heating to 900 K produces substantial reduction of the YSZ(100)-supported CeO2 film,16-18 these results correspond to a highly reduced sample. Also, note that the change in the H2CO/CO selectivity was accompanied by a change in the relative amount of H2O and H2 produced. On the more oxidized surfaces, H2O production dominates, whereas on the highly reduced surface of the last run only H2 production is observed. These results demonstrate that the extent of reduction of the surface or alternatively the oxidation state of the surface cerium cations plays a pivotal role in determining the selectivity for formation of H2CO and CO from adsorbed methoxides. The primary decomposition pathway for methoxide intermediates adsorbed on Ce4+ cations appears to be dehydrogenation to produce H2CO and surface hydroxyl groups. The surface hydroxyl groups either react with additional methoxides to reform CH3OH or react to produce H2O. In contrast, methoxides adsorbed on more highly reduced portions of the surface, possibly on Ce3+ sites, undergo complete dehydrogenation to CO and H2. Note that although the temperature at which this reaction takes place increases as the surface becomes more highly reduced, both CO and H2 are always produced simultaneously. Vapor-deposited CeO2 thin films on R-Al2O3(0001) substrates have previously been shown to be polycrystalline.21 Theoretical molecular mechanics models27 predict that the (111) orientation is the most stable surface plane of CeO2 and the (110) and (100) surfaces are only slightly less stable. Thus, these planes are likely to be preferentially exposed on the surface of the CeO2/R-Al2O3(0001) sample. The TPD results show that a freshly prepared CeO2/R-Al2O3(0001) sample was much less active for both the dissociative adsorption of methanol and the oxidation of surface methoxides to formaldehyde than the CeO2/ YSZ(100) sample. The complete dehydrogenation of methoxides to CO did not occur on the freshly prepared CeO2/ R-Al2O3(0001) sample. Because previous XPS results have shown that the cerium cations in the freshly grown R-Al2O3(0001)-supported ceria thin film are predominantly in the +4 oxidation state,18 this result provides further support for the conclusion that formaldehyde production results from the reaction of methoxide intermediates adsorbed on surface Ce4+ cations. In subsequent CH3OH TPD runs with the CeO2/R-Al2O3(0001) sample, the amount of CH3OH that adsorbed increased substantially and both H2CO and CO production were observed. As explained previously, CO production was again accompanied by the production of H2. These changes can be attributed to the creation of surface oxygen vacancies along with the reduction of a portion of the surface cerium cations from +4 to +3. These results are consistent with those obtained for CeO2/YSZ(100) and again suggest that the complete dehydrogenation of surface methoxide to CO takes place at surface Ce3+ sites. Conclusions The TPD results for the three different samples used in this study demonstrate that the reaction of CH3OH on ceria is highly structure sensitive. The presence of surface oxygen vacancy sites and/or exposed surface Ce3+ cations has a dramatic effect on the surface reactivity. On nearly stoichiometric surfaces, CH3OH adsorbs dissociatively on oxygen vacancy sites to produce surface methoxides. These species undergo oxydehydrogenation to formaldehyde and water at 680 K on CeO2(111) and at 600-630 K on CeO2 films supported on YSZ(100) and R-Al2O3(0001). On highly
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reduced ceria surfaces with exposed Ce3+ cations (e.g., CeO2/YSZ(100) annealed at 900 K), the primary reaction pathway for adsorbed methoxide intermediates is complete dehydrogenation to CO and H2 which occurs near 670 K. Acknowledgment. This work was supported by both the Department of Energy, Office of Basic Energy Sciences
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(Grant No. DE-FG02-96ER14682), and the National Science Foundation (Grant No. CTS-9712774). Some facilities used in this work were also partially funded by the National Science Foundation through the MRSEC program (Grant No. DMR00-79909). LA001729O
